A pressurized-water reactor installation commonly comprises a pressure vessel which contains the core, and a main reactor coolant system formed by at least one main coolant loop, conventionally three or four loops, the loop being formed by a steam generator and a main coolant pump and main coolant piping interconnecting the vessel, pump and generator. The loop contains water coolant and is connected with a pressurizer which maintains a normal operating pressure on the water to prevent it from boiling in the loop and pressure vessel, the pressurized water functioning to cool the core and carry the heat to the steam generator's heat exchanger, the water continuously circulating under the force of the main coolant pump.
If the loop opens up, such as might happen by one of its necessary connections rupturing, the water pressure drops, correspondingly dropping the pressure in the pressure vessel, and depriving the core of cooling. The installation includes a protective system which is expected to scram the core in case of such an accident.
In addition, the reactor is provided with an emergency core cooling system formed by a supply of emergency water which is pressurized at a lower pressure than the normal operating pressure of the main coolant loop and which connects with the loop through a check valve held closed by the normal pressure differential and which automatically opens when the pressure of the emergency water supply exceeds the normal operating pressure of the loop, this occuring in the event of the described type of accident to the loop and resulting in the emergency water being injected into the loop and, therefore, into the pressure vessel to flood the core. The supply of emergency water normally includes a supply of borated water which is injected first to shut down the core promptly, together with a supply of water which follows, the emergency water escaping from the ruptured coolant loop, falling into the sump of the reactor building conventionally enclosing the installation, and being pumped from the sump back into the loop, normally through a heat exchanger. In this way, in case of a loss-of-coolant accident, a pressure vessel is promptly supplied with an initial flooding of borated water, followed by a circulation of emergency water.
Such a loss-of-coolant accident is considered to be only hypothetically possible. In the same sense, it can be contended that the main coolant pump might fail, this pump necessarily comprising a casing containing an impeller connected by a shaft to an electric motor. Mechanical failure can be argued as being conceivable, and, of course, an electric power failure would result in putting the pump out of operation.
If the main coolant pump ceases to function for any reason, the coolant circulation through the main coolant loop reduces to a substantial extent and possibly might stop altogether. In such an event, the core would normally be scrammed by activation of the reactor protective system, but the emergency core cooling system, depending on the presssure differential at the check valve, would not go into operation because the coolant pressure in the main coolant loop would not be reduced. As a hypothetically possible accident, the protective system might fail with the result that the core continues to operate at its full rated power, but deprived of the coolant circulation relied on to carry the heat from the core through the steam generator for heat removal and back through the pressure vessel to the core. In such an event, the coolant pressure would rapidly rise to a degree which might result in rupturing the main coolant pipes. Although the emergency core cooling system would then go into action, the pipe would remain ruptured and require repair. If, in addition, almost inconceivably the emergency core cooling system failed to operate, a core melt-down might be the result.
The pressurizer for the main coolant loop is formed by a tank containing water pressurized by steam above the water in the tank, a water spray in the tank serving to condense the steam and drop the pressurizer pressure and heating the water serving to increase the pressure, all as required to maintain the pressure in the main coolant loop at its normal operating pressure. The pressurizer is provided with a pressure-relief valve for its steam space, but when operated, this valve provides for a flow passage of very small cross-sectional area, and can provide only a very small relief for the pressure in the main coolant loop should it increase due to a failure of the main coolant pump. Typically, the flow rate of this pressure relief valve corresponds to only a few percent of the nominal power rating of the reactor core. The emergency core cooling system is combined with a core decay heat removal system which circulates water coolant through the pressure vessel when the core is shutdown as for refueling, and the pressurizer pressure-relief valve is provided only for use at such times.